The research examines the development of selective attention in school-aged children. Our previous research, using a simple speeded classification procedure, has shown that young school-aged children are particularly vulnerable to interference by irrelevant stimuli. The present research examines the nature of this distractibility. Experiment 1. in a developmental design, determines whether the interference effect is constant regardless of the difficulty of the baseline task, or whether it is proportional to task difficulty. This issue is particularly important for theoretical accounts of the development of selective attention. Experiment 2 examines the degree to which previous experience with the irrelevant stimuli produces internal competing responses which interfere with performance. Experiment 3 determines, as a function of age, whether the interference effects can be eliminated with practice. Experiment 4 examines ways in which children can overcome the interference effect, particularly with respect to the novelty and dimensional structure of the irrelevant stimuli.